Finite Element Analyses On Damage Mechanisms Of Flexible Woven Composites Under Tear And Stab Loading

Textile structural flexible composites have a great potential for use in applications such as bullet-proof/stab-resistant materials, lightweight architectures and oil pipeline structures. During their use, the flexible composites are likely to suffer from tear and stab damages as compared to rigid textile structural composites. The aim of studying tearing and stab failure behaviors of flexible composites is to aid in designing of a flexible composite with high tear and stab resistance. This paper will report the tearing and stab behaviors of woven fabrics and flexible composites in experimental and numerical simulations. Numerical simulations were developed based on the microstructure parameters of woven fabrics and flexible composites to show the tear and stab failure mechanisms on mesoscale.From the experiments, tearing and puncture load-displacement curves, failure processes and failure morphologies were obtained. The trapezoid tearing tests were carried out according to American Society for Testing and Materials (ASTM) D885-07standards. The circle-shaped sample was adopted in the puncture test while the movement velocity of the jaw for both tearing and puncture tests was100mm/min. The tearing and puncture strength as well as damage morphologies obtained from experiments of two kinds of specimens were compared. The differences in tearing and stab performances between woven fabrics and flexible composites revealed the influence of structural parameters.In finite element analysis (FEA), microstructure geometrical models of both woven fabrics and flexible composites were constructed based on microstructural parameters of two specimens in ABAQUS/CAE. The mechanical parameters of yarns and coating resin were incorporated into the finite element (FE) model to calculate dynamic tear and stab failure processes. The stress distribution as well as the failure and slippages of fiber tows at local damage area could be visually displayed in the FE results. This will be helpful in analyzing the influence of microstructural parameters on the tear and stab performances.The main conclusions are as follows:(1) Tear behaviors of woven fabrics:We found out that the main factors influencing the tear performance of woven fabrics are:tensile properties of warp and weft yarns, frictional coefficient, warp and weft density as well as fabric compactness parameters. The shape, size and stress distribution of tearing delta region were dramatically influenced by factors mentioned above. The slippage characteristics between yarns, damage morphologies of tearing region and tearing strength differ from each specimen during the tearing process.(2) Tear behaviors of flexible composites:There was no much difference in tearing strength between flexible composites and woven fabrics. It was obvious that the tearing damage area of the woven fabric is greater than that of the flexible composite. In addition, the fracture of weft yarns in the woven fabric was irregular while the pre-slit was propagated along a straight line strictly in the flexible composite. Coating resin increases the connection points between yarns and this prevents relative slipping between yams which leads to a smaller tearing triangular zone. However, the coating resin itself also contributes to tearing strength of flexible composite. The two contrary effects offset each other resulting in slight difference of tearing strength between woven fabrics and flexible composites.(3) Stab behaviors of woven fabrics:Warp and weft density, tensile properties of yarns as well as frictional coefficient between the yarns are the major factors that affect stab behaviors of woven fabrics. The stab damage process of woven fabrics can be divided into three stages:(â… ) initial stage:the steel penetrator comes in contact with the fabric. The yarns which are directly in contact with the penetrator will bear the tensile load and will be stretched more.(â…¡) intermediate stage:with the increase of the penetration depth, some yarns reach the breaking strength and fail. Meanwhile, the deflection of the fabric gradually increases.(â…¢) final stage:the yarns which are directly in contact with the penetrator will break and slip from the penetrator surface. The un-simultaneous breakages of the yarns leads to the fluctuation of the stab load-deflection curves.(4) Stab behaviors of flexible composites:The stab strength of the flexible composite is significantly higher than that of the woven fabric and also, the stab damage area of the flexible composite is much smaller than that of the woven fabric. The significant improvement of the stab strength is mainly due to the functions of coating resin which include preventing slippages between warp and weft yarns, protecting fibers from direct contact with the penetrator and distributing load concentration to more fibers. Although resins generally have low mechanical properties compared to those of fibers, the coating resin significantly improves the stab properties of the flexible composite.The good agreement between experimental and FE results validates the accuracy of FE models. The numerical simulation will be helpful in predicting tear and stab damage mechanisms on the microstructural level as well as designing flexible composites with high tear and stab resistances. The designing parameters contain specifications of woven fabrics, mechanical properties of yams and coating resin. In practical engineering, the microstructural factors, fiber and coating resin materials can be determined according to the target strength.